The present invention relates to a receiver with a frequency offset correcting function used for radio digital data transfer such as that for an automobile telephone.
Prior to description of the prior art, description is made for a technological background concerning the present invention.
FIG. 14 shows a model of a channel with intersymbol interference (ISI) therein.
The model expresses a channel with a finite impulse response (FIR) filter. In the model, a received signal is a synthesized signal synthesized from a lead signal with the output thereof directly received and a delay signal with the output thereof received with a delay due to its reflection or so.
In the figure, a time difference between delay signals is given by a delay circuit DELAY comprising L-segmented shift register. The lead signal is obtained by multiplying a transmitted signal In by channel impulse response (CIR) c0,n as a tap coefficient with a multiplier MULT0. Herein, a subscript n of CIR c0,n indicates a time of data received during TDMA communications.
Also, delay signals are obtained by multiplying delayed transmitted signals Inxe2x88x921 to Inxe2x88x92L by tap coefficients c1, n to cL,n with multipliers MULT1 to L respectively. Then, outputs of delay signals from the multipliers MULT0 to L are summed up by a summing device SUM, and an adder (ADD) adds noise Wn to the summed wave outputted from the summing device (SUM) to output the added wave as a received signal rn.
When intersymbol interference (ISI) is not present in the channel, the received signal rn is expressed with the following equation.
rn=c0,n In+Wnxe2x80x83xe2x80x83(1)
In this case, c0, n is a known value, so that a transmitted signal In can easily be estimated from a received signal rn on condition that a noise Wn is small.
By the way, according to the model in FIG. 14, when a transmitted sequence of {In} is transmitted to the channel, this transmitted sequence undergoes intersymbol interference (ISI) in addition to additive white Gaussian noise Wn in the channel. Accordingly, the received signal rn includes not only a time n but also a transmitting sequence In in the past before that time. The received signal at this time is expressed with the following equation:
xe2x80x83rn=xcexa3ci,n Inxe2x88x92i+Wnxe2x80x83xe2x80x83(2)
wherein the sum xcexa3 is obtained for values of i=0, . . . , L, and L indicates a time length affected by intersymbol interference (ISI), namely a channel memory length.
In the model of the channel shown in FIG. 14, the transmitted sequence In includes a range from time n to time (nxe2x88x92L). An equalizer is often used for the channel described above as a device for estimating a transmitted sequence In from a received signal rn.
Also, when there is frequency offset xcex94xcfx89 generated due to a difference between a local oscillator of a transmitter and a local oscillator of a receiver, a received signal is expressed with the following equation:
rn=xcexa3ci,n Inxe2x88x92iexp (xcex94xcfx89n+xcex80)+Wxe2x80x2nxe2x80x83xe2x80x83(3)
wherein xcex80 is an initial phase, and Wxe2x80x2n is expressed with the following equation:
Wxe2x80x2n=Wnexp (xcex94xcfx89n+xcex80)xe2x80x83xe2x80x83(4)
As described above, the performance of a receiver is deteriorated due to distortion caused by frequency offset xcex94xcfx89 in addition to intersymbol interference (ISI). And for this reason, the receiver needs to correct the intersymbol interference (ISI) and also the distortion caused by frequency offset xcex94xcfx89.
Next description is made for an example of a receiver with a frequency offset correcting function based on the conventional technology.
FIG. 15 is a block diagram showing the conventional type of receiver for correcting frequency offset. The receiver in this example is the same as that described in xe2x80x9cMethod and Device for Compensating Carrier Frequency Offset in TDMA Communication System (Japanese Patent Laid-open Publication No. HEI 6-508244)xe2x80x9d disclosed by Lin Yuphan et al.
In FIG. 15, designated at the reference numeral 211 is a CIR estimating circuit for estimating CIR according to a training sequence in a received signal, at 212 a phase deviation detecting circuit for computing a phase deviation according to the CIR estimated value estimated by the CIR estimating circuit 211 and a tail bit described later of the received signal, at 213 an averaging circuit for averaging the phase deviations outputted from the phase deviation detecting circuit 212 and computing a frequency offset estimated value, at 214 a frequency offset correcting circuit for correcting the received signal rn according to the frequency offset estimated value outputted from the averaging circuit 213, and at 215 an equalizer for equalizing the received signal rxe2x80x2n corrected by the frequency offset correcting circuit 214 according to the CIR estimated value outputted from the CIR estimating circuit 211, andestimating the transmitted data sequence.
FIG. 16 shows a burst B1 of received signals received during TDMA communications based on the conventional technology shown in FIG. 15.
In the figure, this burst B1 comprises a training sequence B11, data sequence B12, B13, and tail bits B14, B15, and the training sequence B11 and the tail bits B14, B15 are known in the receiver side.
Next description is made for operations in the example based on the conventional technology with reference to FIG. 15 and FIG. 16.
At first, the CIR estimating circuit 211 computes, when having received a received signal rn, CIR estimated values g0, g1, . . . , gL according to the training sequence B11 in the received burst B1 as shown in FIG. 16 as well as to the training sequence having previously been known in the receiver side.
Then, the phase deviation detecting circuit 212 first computes a phase deviation xcfx86m with the following equation according to the CIR estimated values g0, g1, . . . , gL estimated with the known training sequence in the received burst B1 by the CIR estimating circuit 211 as well as to the known tail bits Inxe2x88x92L, Inxe2x88x92L+1, . . . , In. It should be noted that a subscript m in the equation indicates a phase deviation in m-th received burst.
sn=xcexa3giInxe2x88x92ixe2x80x83xe2x80x83(5)
xcfx86m={Im[rn]xc2x7Re[sn]xe2x88x92Im[sn]xc2x7Re[rn]}/{ABS[rn]xc2x7ABS[sn]}xe2x80x83xe2x80x83(6)
wherein the sum xcexa3 is obtained for i=0, . . . , L. It should be noted that L indicates, as shown in the channel model in FIG. 14, a time length affected by intersymbol interference (ISI), namely a channel memory length, and corresponds to the number of stages in the delay circuit DELAY. Also, in the equation, designated at the reference sign sn is a replica (estimated value) of a received signal, at Re[a] a real part of a complex number a, at Im[a] an imaginary part of the complex number a, and at ABS[a] an absolute value of the complex number a respectively.
Further, the phase deviation detecting circuit 212 computes a phase deviation per symbol xcex94xcfx86m through the following equation according to the phase deviation xcfx86m as described above, and outputs a result of the computing to the averaging circuit 213.
xcex94xcfx86m=xcfx86m{2/(Mxe2x88x921)}xe2x80x83xe2x80x83(7)
wherein M indicates a total number of symbols of a received burst B1.
Then, the averaging circuit 213 averages the phase deviation per symbol xcex94xcfx86m estimated for each burst B1, and outputs a result of averaging to the frequency offset correcting circuit 214 as a frequency-offset estimated value xcex94xcfx89m.
The frequency offset correcting circuit 214 corrects frequency offset of a received signal rn through the following equation according to the frequency-offset estimated value xcex94xcfx89m.
rxe2x80x2n=rnexp (xe2x88x92jxcex94xcfx89mn)xe2x80x83xe2x80x83(8)
The equalizer 215 estimates transmitted data sequence according to the received signal rxe2x80x2n having been subjected to offset correction outputted from the frequency offset correcting circuit 214 as well as to the CIR estimated value outputted from the CIR estimating circuit 211, and outputs a result of the decision as a decision value.
However, in the receiver with the conventional type of frequency offset correcting function, known data such as tail bits other than the training sequence is required to compute a frequency-offset estimated value, and also a length of tail bits in a received signal is generally required to be longer than a memory length L of the channel, so that transmission efficiency is worse in turn by the length required for the tail bits.
In the example based on the conventional technology, a phase deviation is computed according to the CIR estimated value, the tail bits and the received signal, so that the phase deviation to be detected largely varies with noises. Accordingly, in order to estimate frequency offset with sufficient precision, it is required to suppress variation by making a time constant larger for averaging phase deviations, and for this reason, when frequency offset varies with time, it is difficult to follow the variation in the method described above.
Further, as diversity reception is not performed in the example based on the conventional technology, an error rate in decision is higher as compared to the case where diversity reception is performed.
The present invention has been made for solving the problems as described above, and it is an object of the present invention to provide a receiver with a frequency offset correcting function which has improved the capabilities of being excellent in transmission efficiency without requiring known data other than a training sequence, precisely estimating time-varying frequency offset, and further enabling performance of diversity reception and determination of data at a low error rate.
To achieve the object as described above, the present invention comprises a frequency offset correcting means for receiving a received signal as well as a frequency-offset estimated value and correcting phase rotation due to frequency offset of the received signal according to the frequency-offset estimated value; a first channel impulse response estimating means for estimating channel impulse response at a first position of the corrected received signal according to a known training sequence included in the received signal corrected by the frequency offset correcting means; a determining means for determining the received signal corrected by the frequency offset correcting means according to the channel impulse response estimated value at the first position thereof estimated by the first channel impulse response estimating means; a second channel impulse response estimating means for estimating channel impulse response at a second position apart from the first position of the corrected received signal according to the received signal corrected by the frequency offset correcting means, the channel impulse response estimated value at the first position thereof estimated by the first channel impulse response estimating means, and to the value determined by the determining means; and a frequency-offset estimated value computing means for computing a frequency-offset estimated value of the received signal according to the channel impulse response estimated value at the first position thereof estimated by the first channel impulse response estimating means as well as to the channel impulse response estimated value at the second position thereof estimated by the second channel impulse response estimating means, and outputting the computed value to the frequency offset correcting circuit. Therefore, with this invention, different channel-impulse estimated values at the first and second positions are obtained according to the training sequence having been known in the received signal, and a frequency-offset estimated value is computed according to those phase deviations, so that frequency offset of a received signal can be corrected and also data can be determined without using known data such as tail bits other than the training sequence. As a result, tail bits are not needed as a burst structure of a received signal, so that transmission efficiency is improved, and also a phase deviation can be computed not according to an estimated value (replica) of the received signal computed only with tail bits in a transmission sequence but according to channel impulse response estimated by sufficiently suppressing a noise element with an appropriate algorithm (e.g., LMS algorithm), and for this reason a phase deviation to be detected does not largely varies with noises, time-varying frequency offset can be compensated with high precision, and data can be determined at a low error rate.
The another present invention comprises a frequency offset correcting means for receiving a received signal as well as a frequency-offset estimated value and correcting phase rotation due to frequency offset of the received signal according to the frequency-offset estimated value; a first channel impulse response estimating means for estimating channel impulse response at a first position of the corrected received signal according to a known training sequence included in the received signal corrected by the frequency offset correcting means; a determining means for determining the received signal corrected by the frequency offset correcting means; a second channel impulse response estimating means for estimating channel impulse response at a second position apart from the first position of the corrected received signal by updating the channel impulse response estimated value at the first position according to the received signal corrected by the frequency offset correcting means, the channel impulse response estimated value at the first position thereof estimated by the first channel impulse response estimating means, and to the value determined by the determining means; and a frequency-offset estimated value computing means for computing a frequency-offset estimated value of the received signal according to the channel impulse response estimated value at the first position thereof estimated by the first channel impulse response estimating means as well as to the channel impulse response estimated value at the second position thereof estimated by the second channel impulse response estimating means, and outputting the computed value to the frequency offset correcting circuit; and the determining means determines the received signal corrected by the frequency offset correcting means, at first, according to the channel impulse response estimated value at the first position thereof estimated by the first channel impulse response estimating means, and determines the received signal corrected by the frequency offset correcting means, after the second time and on, according to a value updated from the channel impulse response estimated value at the first position thereof by the second channel impulse response estimating means. For this reason, with this invention, determination of the received signal after the second time and on is made according to values obtained, by successively updating the channel impulse response estimated value at the first position, outputted from the second estimating means, so that, even when the channel impulse response estimated value varies with time, the variation can be followed, and data can be determined at a low error rate.
The another present invention comprises a first channel impulse response estimating means for estimating channel impulse response at a first position of the received signal according to a known training sequence included in the received signal; a determining means for determining the received signal according to the channel impulse response estimated value at the first position thereof estimated by the first channel impulse response estimating means; a second channel impulse response estimating means for estimating channel impulse response at a second position apart from the first position of the received signal according to the received signal, the channel impulse response estimated value at the first position thereof estimated by the first channel impulse response estimating means, and to the value determined by the determining means; a frequency-offset estimated value computing means for computing a frequency-offset estimated value of the received signal according to the channel impulse response estimated value at the first position thereof estimated by the first channel impulse response estimating means as well as to the channel impulse response estimated value at the second position thereof estimated by the second channel impulse response estimating means; and a local oscillator correcting means for correcting a frequency from a local oscillator according to the frequency-offset estimated value computed by the frequency-offset estimated value computing means. Therefore, with this invention, a frequency from the local oscillator of the receiver is directly controlled in place of correcting frequency offset of the received signal, so that configuration of the circuit can be simplified.
The another present invention comprises a frequency offset correcting means for receiving a plurality of received signals as well as frequency-offset estimated value, and correcting each phase rotation due to frequency offset for the plurality of received signals according to the frequency-offset estimated value respectively; a first channel impulse response estimating means for estimating each channel impulse response at a first position of the plurality of corrected received signals according to each known training sequence included in the plurality of received signals corrected by the frequency offset correcting means; a determining means for determining the plurality of received signals corrected by the frequency offset correcting means according to the channel impulse response estimated values each at the first position thereof estimated by the first channel impulse response estimating means; a second channel impulse response estimating means for estimating each channel impulse response at a second position apart from the first position of the plurality of corrected received signals according to the plurality of received signals corrected by the frequency offset correcting means, each channel impulse response estimated value at the first position of the plurality of corrected received signals estimated by the first channel impulse response estimating means, and to the value of the plurality of received signals determined by the determining means; and a frequency-offset estimated value computing means for computing frequency-offset estimated value of the plurality of received signals according to each channel impulse response estimated value at the first position thereof estimated by the first channel impulse response estimating means as well as to each channel impulse response estimated value at the second position thereof estimated by the second channel impulse response estimating means, and outputting the computed value to the frequency offset correcting circuit. Therefore, with this invention, a plurality of received signals can be received with a plurality of frequency offset correcting circuits or the like respectively, so that diversity reception can be performed, and data can be determined at a low error rate.
In the present inventions, the frequency-offset estimated value computing means comprises a phase deviation detecting means for detecting a phase deviation between the first position and the second position according to the channel impulse response estimated value at the first position estimated by the first channel impulse response estimating means as well as to the channel impulse response estimated value at the second position estimated by the second channel impulse response estimating means; and an averaging means for averaging phase deviations obtained by computing a phase deviation per symbol according to the phase deviation between the first position and the second position detected by the phase deviation detecting means to compute a frequency-offset estimated value, and outputting the computed value to the frequency offset correcting circuit.
Also, the phase deviation detecting means selects a value of which the absolute value is the maximum among the channel impulse response estimated values at the first position estimated by the first channel impulse response estimating means, and also selects a value corresponding to the channel impulse response estimated value at the first position of which the absolute value is the maximum among the channel impulse response estimated values at the second position estimated by the second channel impulse response estimating means; and detects a phase deviation of the received signal according to the product of complex conjugate of the channel impulse response estimated value at the first position of which the absolute value is the maximum and the channel impulse response estimated value at the second position corresponding to the channel impulse response estimated value at the first position of which the absolute value is the maximum.
Also, the phase deviation detecting means computes each product of the complex conjugate of the channel impulse response estimated value at the first position estimated by the first channel impulse response estimating means and the channel impulse response estimated value at the second position estimated by the second channel impulse response estimating means, and detects a phase deviation of the received signal according to the sum of the products each of which the absolute value is more than a threshold value among the computed products.
Also, the phase deviation detecting means computes each product of the complex conjugate of the channel impulse response estimated value at the first position estimated by the first channel impulse response estimating means and the channel impulse response estimated value at the second position estimated by the second channel impulse response estimating means, selects each product of which the absolute value is more than a threshold value among the computed products, multiplies each of the absolute values to the selected products, and detects the phase deviation of the received signals according to the sum of the multiplied values.
Also, the phase deviation detecting means further quantizes the phase deviation of the detected received signal, and outputs a result of the quantization as a phase deviation.
Also, the determining means comprises a soft-decision equalizer for executing soft-decision of a data sequence in the corrected received signal according to the received signal corrected by the frequency offset correcting means as well as to the channel impulse response estimated value at the first position estimated by the first channel impulse response estimating means; and a hard-decision circuit for executing hard decision of the soft-decision value estimated by the soft-decision equalizer and outputting a result of the decision as a decision value of a data sequence in the received signal. Therefore, with this invention, decision can be made including reliability by the soft-decision equalizer, so that the reliability can be utilized in forward error correction and so on.